Various techniques are known for dispersing hydrophobic photographically useful compounds such as photographic couplers into photographic element layer coating compositions comprising hydrophilic colloids.
Photographic dye forming couplers, as well as other hydrophobic photographically useful compounds, are typically incorporated into a hydrophilic colloid layer of a photographic element by first forming an aqueous dispersion of the couplers and then mixing such dispersion with the layer coating solution. An organic solvent is typically used to dissolve the coupler, and the resulting organic solution is then dispersed in an aqueous medium to form the aqueous dispersion.
The organic phase of these dispersions frequently includes high boiling or permanent organic solvents, either alone or with low boiling or partly water soluble solvents which are removed after dispersion formation. Permanent high boiling solvents have a boiling point sufficiently high, generally above 150.degree. C. at atmospheric pressure, such that they are not evaporated under normal dispersion making and photographic layer coating procedures. Permanent high boiling coupler solvents are primarily used in the conventional "oil-protection" dispersion method whereby the organic solvent remains in the dispersion, and thereby is incorporated into the emulsion layer coating solution and ultimately into the photographic element.
The conventional "oil in water" dispersion method for incorporating hydrophobic couplers is described, e.g., in U.S. Pat. No. 2,322,027 by Jelly and Vittum. In such conventional process, the coupler is dissolved in a high boiling water immiscible solvent, mixed with aqueous gelatin, and dispersed using a colloid mill or homogenizer. The presence of the high boiling solvent provides a stable environment for the hydrophobic coupler, as well as generally increasing the reactivity of the coupler upon photographic processing.
Cyan image dye forming couplers of Formula I below wherein R.sub.a is an alkyl group, R.sub.b is a ballast group and X is hydrogen or a coupling-off group, have been found to be very useful in color photographic elements. ##STR3##
In general, such couplers are relatively easy to synthesize and therefore have low manufacturing costs. Phthalic acid esters such as dibutylphthalate have been found to be very useful as coupler solvents in combination with couplers of Formula I. These coupler solvents enable these couplers to provide high reactivity with oxidized color developer. Such solvents also shift the cyan image dye formed from these couplers bathochromically (to longer wavelengths), which is desirable for color reproduction. They also provide cyan image dyes with adequate light and dark stability. Dibutylphthalate, however, has been identified as possessing potentially undesirable biological properties. Hence, it is desired to identify alternative solvents to be used with these cyan couplers, which are more benign in terms of their potential health and environmental effects, but still maintain all of the desirable photographic properties.
Alternative coupler solvents which deliver all of the desired features with these couplers must meet a fairly specific set of criteria. In order to deliver high coupler reactivity, alternative solvents must have sufficiently low viscosity. It is also necessary for potential alternative solvents to have sufficient polarity to shift the cyan dye hue bathochromically. It is also important for the solvent to be relatively hydrophobic so that most of it remains present in the layer in which it was coated and is not removed from the coating to an appreciable extent during processing in order to provide good dark stability of cyan image dyes formed from these couplers.
The selection of an alternative solvent for use with cyan dye forming couplers of Formula I has accordingly required significant exploration and research in order to provide a coupler dispersion which provides desired functionality. It would be desirable to provide a silver halide photographic element which contains a cyan image forming layer comprising a cyan coupler of Formula I and a coupler solvent which meets such objectives.